HDFS-13493. Reduce the HttpServer2 thread count on DataNodes. Contributed by Erik Krogen.

(cherry picked from commit cddbbe5f690e4617413f6e986adc6fa900629f03)

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/web/DatanodeHttpServer.java

@@ -89,6 +89,13 @@ public class DatanodeHttpServer implements Closeable {
   private InetSocketAddress httpsAddress;
   static final Log LOG = LogFactory.getLog(DatanodeHttpServer.class);
 
+  // HttpServer threads are only used for the web UI and basic servlets, so
+  // set them to the minimum possible
+  private static final int HTTP_SELECTOR_THREADS = 1;
+  private static final int HTTP_ACCEPTOR_THREADS = 1;
+  private static final int HTTP_MAX_THREADS =
+      HTTP_SELECTOR_THREADS + HTTP_ACCEPTOR_THREADS + 1;
+
   public DatanodeHttpServer(final Configuration conf,
       final DataNode datanode,
       final ServerSocketChannel externalHttpChannel)
@@ -97,7 +104,12 @@ public class DatanodeHttpServer implements Closeable {
     this.conf = conf;
 
     Configuration confForInfoServer = new Configuration(conf);
-    confForInfoServer.setInt(HttpServer2.HTTP_MAX_THREADS_KEY, 10);
+    confForInfoServer.setInt(HttpServer2.HTTP_MAX_THREADS_KEY,
+        HTTP_MAX_THREADS);
+    confForInfoServer.setInt(HttpServer2.HTTP_SELECTOR_COUNT_KEY,
+        HTTP_SELECTOR_THREADS);
+    confForInfoServer.setInt(HttpServer2.HTTP_ACCEPTOR_COUNT_KEY,
+        HTTP_ACCEPTOR_THREADS);
     int proxyPort =
         confForInfoServer.getInt(DFS_DATANODE_HTTP_INTERNAL_PROXY_PORT, 0);
     HttpServer2.Builder builder = new HttpServer2.Builder()